1. Field of the Invention
The present invention relates to sound quality improvement for an apparatus using a digital interface for audio and video data such as the HDMI (High Definition Multimedia Interface) or the like.
2. Description of the Related Art
Conventionally, the HDMI is widely used as an interface capable of transferring digital video and audio signals while protecting the copyright thereof. Especially for BD-ROM and the like, high-bit-rate multi-channel audio signals are adopted. As a result, conventional audio interfaces using coaxial optical cables cannot transfer such data because of capacity shortage. In order to transfer audio information at a high bit rate, the HDMI is the only available means. In accordance with this, the sound quality of the HDMI has been a recent problem to be solved. It is known that when a digital audio signal is used, the sound quality is deteriorated due to audio clock jitter. According to the HDMI, an audio clock is not directly transferred; instead, all the video and audio information is transferred in synchronization with a video clock. At the same time, two parameters (N value and CTS value) representing the frequency ratio of the audio clock and the video clock are transferred, and the apparatus receiving the signals reproduces the audio clock by PLL based on the information. Due to such a structure, jitter caused by the PLL is one reason for the deterioration of the sound quality. Recently, a method of controlling the parameter N such that the clock is reproduced by the PLL with low jitter has been introduced (see Japanese Laid-Open Patent Publication No. 2009-105889).
A recently known apparatus developed for preventing the deterioration of the sound quality as much as possible and improving the sound quality has two HDMI output ports which are respectively used exclusively for video signals and used exclusively for audio signals. At the output used exclusively for audio signals, the video signals are made completely black, so that the video signals are preventing from disturbing the audio signals in the audio signal receiving apparatus.